Modeling prediction via experimental testification: Conductive MOFs are promising for supercapacitors with ionic liquid electrolytes

A new class of supercapacitor system is investigated theoretically and experimentally, which exploits electronically conducting metal-organic frameworks (MOF) for electrode materials and ionic liquids as electrolytes.

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Electrical double-layer capacitors (EDLCs) have merits such as high-power density and long cycle times, as they store the electrical energy through electrolyte ions adsorption at electrified electrode surface. Among electrode materials, state-of-the-art conductive porous metal organic frameworks (MOFs) could be of great interest. Their scaffold-shaped structure could not only bring a very high specific surface area (SSA) with custom-built pore size distribution, which help to maximize the specific energy density, but also facilitate the ion transport during charging/discharging process (high power density). With respect to electrolytes, room-temperature ionic liquids (RTILs) have become emerging due to their advantaged features. Among them are wide electrochemical window, excellent thermal stability, non-volatility, etc. Therefore, EDLCs with MOF electrodes and RTIL electrolytes could potentially be very promising for future electrical energy storage devices.

In this study, we focused on the theoretical exploration of energy storage and power delivery of such MOF-RTIL type supercapacitors, in order to reveal how much benefit that they could in principle offer on those fronts, as compared to widely-used porous carbon materials. We present a ‘computational microscopy’ analysis (constant potential molecular dynamics simulations) of the structure and performance of conductive MOF electrodes in supercapacitors with room temperature ionic liquids. The molecular modeling predicts the characteristic shapes of the potential dependence of electrode capacitance, relying on the structure of MOF electrodes and particularly how ions transport and reside in MOFs under polarization. Transmission line model was adopted to characterize the charging dynamics process and build up a bridge to evaluate the capacitive performance of practical supercapacitor devices at macroscale from the simulation-obtained data at nanoscale. Such nanoscale-to-macroscale analysis revealed that these MOF/RTIL-based cells could exhibit performance superior to most carbon-based devices. These ‘computational microscopy’ results were further testified by macroscopic electrochemical measurements: the experimental areal capacitance and equivalent series resistance obtained in experiment quantitatively agree with modeling predictions, signifying that our molecular simulation could well represent the real system of MOF-based supercapacitor.

Therefore, our findings and analyses provide not only molecular insights into the preferred structures of MOF for achieving these results, but a promising avenues for designing supercapacitors with both high energy and power densities.

All the details of our work can be read in our article in Nature Materials: https://www.nature.com/articles/s41563-019-0598-7

 

Guang Feng

Professor, Huazhong University of Science and Technology

Guang Feng is a professor in Huazhong University of Science and Technology. He received his Ph.D. degree in 2010 from Clemson University, USA as the Outstanding Student in the Doctoral Degree Program awardee in Department of Mechanical Engineering. During 2010 to 2014, he worked in Vanderbilt University and The Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center as a postdoctoral research associate and then a research assistant professor. By November 2013, he became a professor in State Key Laboratory of Coal Combustion and School of Energy and Power Engineering in Huazhong University of Science and Technology, China, via the Hubei Provincial 100-Talents Program. He has published 3 book chapters and more than 70 papers in peer-reviewed journals (e.g., Nature Materials, Nature Communications, Physical Review X, Advanced Energy Materials, ACS Nano, Nano Energy, Nano Letters, etc.). His current research interests are focused on study of micro-/nano-scale interface and transport phenomena in applications of electrical energy storage, capacitive deionization for desalination and water treatment, CO2-EOR, shale gas, and drug delivery. He is a Fellow of the Royal Society of Chemistry, and right now serves as an editorial board member of ChemElectroChem and a young editorial board member of Green Energy & Environment. More information about him is available at http://itp.energy.hust.edu.cn.

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